Write a line-based text editor. The command syntax is similar to the Unix line editor ed. The internal copy of the file is maintained as a linked list of lines. To be able to go up and down in the file,...

Write a line-based text editor. The command syntax is similar to the Unix line editor ed. The internal copy of the file is maintained as a linked list of lines. To be able to go up and down in the file, you have to maintain a doubly linked list. Most commands are represented by a one-character string. Some are two characters and require an argument (or two). Support the commands shown in Figure 31.